SEATTLE — As the holiday shopping season revs up, Amazon.com Inc. is adding another weapon to its competitive arsenal: Sunday delivery.

The online retail giant has cut a deal with the Postal Service to extend delivery of packages to customers to every day of the week. That means Amazon Prime customers who pay $79 a year to get free two-day shipping on millions of products can order an item Friday and have it arrive before the weekend is over. And Sunday delivery will be available to customers who don’t subscribe to Prime as well.

Sunday deliveries of Amazon products will start this weekend, but only to customers in the Los Angeles and New York metropolitan areas. The company, which needs to build up the technical capability at warehouses elsewhere, plans to work with the USPS to roll out the service to “a large portion of the U.S. population” next year.

Details of the agreement have been sealed with the Postal Regulatory Commission, an independent agency with regulatory oversight over the Postal Service, according to USPS spokeswoman Sue Brennan.

It’s the first time the Postal Service has worked with a retailer to enable Sunday delivery.

“It requires the scale that Amazon has,” said Dave Clark, Amazon’s vice president of worldwide operations and customer service.

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